Do cats cause schizophrenia? Longitudinal study challenges previous findings

Study

“Curiosity killed the cat: no evidence of an association between cat ownership and psychotic symptoms at ages 13 and 18 years in a UK general population cohort,” published in Psychological Medicine, 2017. Abstract available online here.

Overview

In this study, researchers used data from the large-scale Avon Longitudinal Study of Parents and Children (ALSPAC) to investigate the possible relationship between cat ownership in pregnancy and early childhood (measured at 4 and 10 years of age) and psychotic experiences at ages 13 (6,705 survey respondents) and 18 (4,676 survey respondents) [1]. Their analysis suggests that cat ownership in pregnancy and early childhood poses no increased risk of psychotic experiences among young adults.

Key points

These researchers were interested in examining the possible connections between cat ownership in pregnancy and early childhood and psychotic experiences among young adults, in part, as a response to earlier studies that had demonstrated such a link [2,3] (in particular, to schizophrenia) but were “hindered by notable methodological limitations” [1]. And psychotic experiences in adolescence are, as the researchers note, “an established risk factor for later schizophrenia, particularly with respect to psychotic symptoms which emerge or persist in late adolescence” [1].

Earlier investigations likely suffered from small sample sizes, lack of control for confounding variables, selective participation, and recall bias.

Many previous investigations assumed that cat ownership early in life increases the risk of infection with the common parasite Toxoplasma gondii, for which cats (both domestic and wild felids) are the only known “definitive host” (the parasite is able to reproduce sexually and form egg-like spores, called oocysts, in these animals).”

Results

This study revealed no evidence of a relationship between cat ownership in pregnancy and early childhood and psychotic experiences among young adults — thereby challenging the links between cat ownership, T. gondii infection, and mental illness suggested by some previous studies.

Methodology

The large sample size and research design employed in this study offered many advantages over previous research on the topic. The Avon Longitudinal Study of Parents and Children recruited 14,541 pregnant women from Avon (formerly designated as a county in the southwest of England) with a total of 14,701 children eventually enrolled.

The unusually large sample used for this study allowed researchers to consider various cofounding factors (e.g., “ethnicity, maternal academic achievement, and social class”) and examine the impact of missing data in their analysis. Among the key advantages of prospective cohort studies is that they allow researchers to know when “exposure” occurs (e.g., cat ownership) and whether or not that precedes the outcome of interest (e.g., psychotic experiences). They also allow researchers to assign numeric values to risk factors.

As also noted by the authors of a related 2016 study [4], results of this study differ from those of many other on this topic. The researchers note that earlier investigations likely suffered from small sample sizes, lack of control for confounding variables, selective participation (i.e., a non-random sample), and recall bias (i.e., when participants report previous experiences inaccurately).

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References

1. Solmi, F.; Hayes, J.F.; Lewis, G.; Kirkbride, J.B. Curiosity killed the cat: no evidence of an association between cat ownership and psychotic symptoms at ages 13 and 18 years in a UK general population cohort. Psychological Medicine 2017, 1–9.

2. Torrey, E.F.; Yolken, R.H. Could Schizophrenia Be a Viral Zoonosis Transmitted From House Cats? Schizophrenia Bulletin 1995, 21, 167–171.

3. Torrey, E.F.; Simmons, W.; Yolken, R.H. Is childhood cat ownership a risk factor for schizophrenia later in life? Schizophrenia Research 2015, 165, 1–2.

4. Sugden, K.; Moffitt, T.E.; Pinto, L.; Poulton, R.; Williams, B.S.; Caspi, A. Is Toxoplasma Gondii Infection Related to Brain and Behavior Impairments in Humans? Evidence from a Population-Representative Birth Cohort. PLoS ONE 2016, 11, e0148435.